The technical field generally relates to control of selective catalytic reduction (SCR) aftertreatment systems for internal combustion engines. SCR systems present several control challenges for internal combustion engine applications, including for mobile applications. SCR systems include a reduction catalyst and a reducing agent, such as urea or ammonia.
An injector provides the reducing agent to the exhaust stream at a position upstream of the SCR catalyst, and the reducing agent enters the gas phase of the exhaust stream as ammonia. A delay sometimes occurs between the introduction of the reducing agent and the availability of the reductant product, for example injected particles of the reducing agent may need to evaporate into the exhaust stream, hydrolyze from urea to ammonia, and/or thoroughly mix into the exhaust stream for general availability across the SCR catalyst. Additionally, the SCR catalyst may include some ammonia storage capacity. Storage capacity can complicate the controls process, for example by creating additional controls targets (e.g. a storage target), by releasing ammonia unexpectedly (e.g. when a system condition causes a decrease in storage capacity), and/or by adsorbing some of the injected ammonia in an early part of the catalyst thereby reducing the availability of ammonia at a rear portion of the catalyst during catalyst filling operating periods.
The challenges presented by presently available SCR systems are exacerbated by the transient nature of mobile applications. The engine load and speed profile varies during operations in a manner that is determined by an operator and generally not known in advance to the SCR control system. Additionally, available feedback systems suffer from several drawbacks. For example, the concentration of ammonia is difficult to determine in real time. Commercially reasonable ammonia sensors often suffer from cross-sensitivity with NOx, complicating the determination of the amount of ammonia present. Further, ammonia is generally an undesirable constituent of the final exhaust emissions, and further emitted ammonia represents ineffectively utilized reductant that increases operating costs. Therefore it is desirable to operate at a very low or zero ammonia concentration at the SCR catalyst outlet. Ammonia levels at the SCR outlet in many applications are below the reliable level of commercially reasonable sensor detection, especially in aging or marginal ammonia sensors.